Applying a liquid material, in particular a hot melt, by means of an applicator to a substrate by way of a control valve with controllable opening and closing is a technique which is known per se.
So long as the speed of the material relative to the applicator remains below certain values and the space between the pattern parts is not too small, no particular problems occur, despite the fact that, particularly in the case of pneumatically operated valves, for obtaining a well-defined pattern, i.e. not ragged, large-size air supply and discharge ducts are needed in order to be able to supply and evacuate the control air in a short time.
However, the situation changes when the relative speed increases and the space between the pattern parts has to be small. In this case the time which elapses between quickly making the valve close and subsequently quickly making the valve open is too short to obtain a good effect. This problem occurs in particular in the case of pneumatically controlled valves, in the case of which considerable quantities of air have to be supplied or discharged through large-size ducts in order to obtain the desired rapid opening and closing of the valves.
The object of the invention is to provide a solution to this problem. According to the invention, for this purpose use is made of at least two control valves which are connected to the material supply and are made to open and close sequentially.
If, for example, there are two control valves and the first of these two is made to close at the end of its working cycle through rapid evacuation of the control air, and is not in a position to allow material through again very shortly afterwards by being made to open, the function of said first valve is taken over by the second valve, which is made to open at the correct moment; during the open period of this second valve, there is an opportunity for the situation in the first valve to recover, and said first valve is ready for the next working cycle the moment the second valve is made to close. A very rapid and error-free operation is achieved in this way.
Of course, the principle according to the invention can also be applied to a system with more than two control valves--for example three or even four--which are controlled sequentially.
It is also possible to divide these valves into two or more sets, each set comprising at least two valves, and said sets being made to open and close sequentially. For example, it is possible to make an applicator interact with four or six control valves, sub-divided into two or three sets of two or three valves each, so that even in the case of an applicator with a relatively long nozzle a good effect remains guaranteed.
It is pointed out that an applicator with two control valves is known per se from U.S. Pat. No. 4,735,169. In the case of this known device, however, these control valves are made to open and close simultaneously, so that the principle on which the invention is based is not known from this publication.